JPH06332554A - Controlling method for photovoltaic system - Google Patents

Abstract

PURPOSE:To reduce the required withstand voltage of the electronic parts of an inverter when the linkage is secured with a commercial power system and its torelance and to reduce the cost and the scale of a photovoltaic system. CONSTITUTION:A photovoltaic system includes a solar battery, a boosting circuit 40 and an inverter main circuit. When this generation system is linked to a commercial power system, the pulse width modulation control is applied to the inverter main circuit so that the output voltage of the solar battery is nearly maximized. Furthermore, the boosting rate of the circuit 40 is controlled when the input voltage V2 of the inverter maim circuit exceeds a fixed level lower than the withstand voltage, to suppress the rise of the voltage V2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control method for a photovoltaic power generation system used in connection with a single-phase three-wire commercial power system.

[0002]

2. Description of the Related Art A single-phase, three-wire line is economically advantageous in that it has less voltage drop and power loss during load balancing than the single-phase, two-wire system, so that it can supply power to low-voltage customers such as houses. It is used in the majority of low-voltage lighting lines as a power distribution system for supply.

On the other hand, clean energy without environmental pollution, in particular, photovoltaic power generation using a solar cell has been attracting attention due to the increasing awareness of global environmental protection in recent years.
In solar power generation, the generated power fluctuates greatly depending on the amount of solar radiation, so a solar power generation system (solar cell and inverter installed in buildings and households to ensure stable power supply and effective use of surplus generated power It is desirable to use the system consisting of (and) together with the commercial power system. In other words, power is normally supplied to the load by parallel operation of the solar power generation system and the commercial power system, and part or all of the electric power required for the home is covered by the solar power generation, and the power generated by the solar cell is surplus. If so, a so-called reverse power flow to the commercial power system is performed.

Generally, in converting DC power into single-phase AC power, as shown in FIG. 6, four semiconductor devices (for example, IGBTs) Q1 to Q4, which are switching elements having a self-turn-off function, A single-phase bridge type inverter main circuit 22 composed of four feedback diodes D1 to D4 connected in anti-parallel to each of these is used.

In the inverter main circuit 22, as is well known, a set of semiconductor devices Q1 and Q4 and a semiconductor device Q2.
Divided into groups of Q3, each group is alternately opened and closed (switching)
By doing so, a staircase voltage is obtained between the connection point of the semiconductor devices Q1 and Q2 and the connection point of the semiconductor devices Q3 and Q4. Then, by applying an appropriate pulse width modulation (PWM) to the switching signal, the output voltage waveform can be approximated to a sine waveform.

In principle, the input DC voltage of the inverter main circuit 22 may be the peak value of the output AC voltage (about 1.4 times the effective value), but in reality it is the semiconductor device Q.
In consideration of the voltage drops of 1 to 4, the voltage drop of the filter circuit, the temperature characteristics of the DC power supply (solar cell), etc., it is set to a value about twice the output AC voltage (effective value).

The output line type of the inverter main circuit 22 is a single-phase two-wire type. Therefore, conventionally, as shown in FIG. 7A, a photovoltaic power generation system including a solar cell 10 and an inverter 20A uses a single-phase transformer 30 for linear conversion, which is referred to as an AA transformer. It was connected to a three-wire commercial power system. At that time, when the voltage between the neutral line N of the single-phase three-wire line and each of the voltage lines R and S is 100 Vrms, such as a low-voltage light wire for general household use, the inverter 20A
Output voltage is set to 100 Vrms, and the optimum operating voltage (voltage at which the output power becomes maximum) of the solar cell 10 is 200 Vrms.
The thing of about V was used.

[0008]

The interconnection by the transformer 30 is disadvantageous in reducing the size, weight and cost of the solar power generation system. That is, also in the solar power generation system, it is desired to be transformerless like various electric circuit devices.

Therefore, as shown in FIG. 7B, the output voltage of the inverter 20B is set to 200 Vrms and the output terminals of the inverter main circuit 22 are connected to each other without passing through the transformer 30 during the interconnection with the commercial power system. It is conceivable to connect to the voltage lines R and S.

In this case, the solar cell 10 and the inverter 20
Since it is necessary to make the line voltage of the indoor wiring with B lower than the upper limit value 300V defined by the safety standard, the voltage of 400V required as the DC input of the inverter main circuit 22 (double the output voltage) In order to obtain (1), the solar cell 10 whose open circuit voltage (maximum voltage) is lower than 300V is used, and the booster circuit 40 for boosting the output voltage of the solar cell 10 to 400V is provided in the preceding stage of the inverter main circuit 22. .

Here, in a commonly used silicon solar cell, the open circuit voltage is about 1.about.1, which is the optimum operating voltage, although there are some differences between the single crystal system, the polycrystalline system and the amorphous system.
5 times. Therefore, if the optimum operating voltage of the solar cell 10 is about 200 V as in the conventional case, and the step-up ratio of the step-up circuit 40 is "2", the safety standard is met and the transformer without commercial power system is used. It is possible to realize a photovoltaic power generation system that can be interconnected.

However, in such a solar power generation system, the output voltage of the booster circuit 40 is 600 times the open circuit voltage of the solar cell 10 (600 (=
200 × 1.5 × 2) V can be reached. Therefore, the smoothing capacitor C0 for reducing the input ripple of the inverter main circuit 22 and the inverter main circuit 22
Since each of the semiconductor devices Q1 to Q4 is required to have a withstand voltage of 600 V or higher, there arises a problem that the price increase and the size increase of the inverter 20B cannot be avoided. Further, the higher the breakdown voltage of the semiconductor devices Q1 to Q4, the larger the switching loss.

In view of the above-mentioned problems, the present invention aims to reduce the withstand voltage of electronic components constituting an inverter in a transformerless interconnection with a commercial power system, thereby reducing the price and size of a solar power generation system. The purpose is to realize.

[0014]

In order to solve the above-mentioned problems, a control method according to the invention of claim 1 is a solar cell that outputs DC power, and a booster circuit that boosts the DC power by switching control. Upon interconnection with a commercial power system of a photovoltaic power generation system having an inverter main circuit for converting the DC power after boosting to AC power, the output power of the solar cell is almost maximized, A pulse width modulation control is performed on the inverter main circuit, and when the input voltage of the inverter main circuit exceeds a certain voltage lower than the withstand voltage, the booster circuit suppresses an increase in the input voltage of the inverter main circuit. The step-up ratio is adjusted.

In the control method according to the invention of claim 2, when the input voltage of the inverter main circuit exceeds the withstand voltage,
The booster circuit is put into an operation stop state.

[0016]

Operation Normally, the operating point of the solar cell is maintained at or near the optimum operating point by the pulse width modulation control for the inverter main circuit, and the booster circuit converts the output voltage of the solar cell into the voltage required for conversion to AC. Boost and output. At this time,
The boost ratio is almost "2".

On the other hand, for example, in the transition period from the start of operation of the photovoltaic power generation system to the stable operation, when the amount of solar radiation changes rapidly, the output voltage of the solar cell approaches the open circuit voltage, and Accordingly, when the input voltage of the inverter main circuit approaches a constant voltage lower than the withstand voltage, the booster circuit reduces the boosting ratio so as to suppress the increase of the input voltage of the inverter main circuit.

[0018]

1 is a block diagram showing a state in which a photovoltaic power generation system 1 according to an embodiment of the present invention is connected to a commercial power grid 2.

The solar power generation system 1 is composed of a solar cell 10 having an optimum operating voltage of about 200 V and an inverter 20 of a voltage type current control system, and a power distribution device 4 including an unillustrated integrating power meter, a distribution board and the like. Via a single-phase three-wire commercial power system (voltage between voltage lines is 200 Vrms, frequency is 50/60 Hz) 2. A load Z such as various electric appliances is connected to the distribution line 3.

The inverter 20 includes a noise filter 21 on the input side, a chopper type booster circuit 40 for boosting the output voltage V1 of the solar cell 10, a smoothing capacitor C0 having a withstand voltage of 550 V, and an output voltage V2 of the booster circuit 40 of 200 Vrms.
Single-phase bridge type inverter main circuit 22 for converting into AC voltage, inverter control section 23 for performing PWM switching control of the inverter main circuit 22, and inverse L-shaped output filter for obtaining an output with less harmonic components 24,
Relay 25, output side noise filter 26, circuit breaker 2
7, the current transformer 28 for detecting the output current, and the phase and waveform of the voltage at the interconnection point Px between the photovoltaic power generation system 1 and the commercial power system 2 (that is, the system voltage on the distribution line 3). It is composed of a transformer 29 and the like.

In the relay 25, the system voltage (here, the voltage between the neutral line and one of the voltage lines) is the upper limit value 10 within a specified range.
When the voltage reaches 7 (101 + 6) volts, the inverter control unit 23 cuts off the connection, thereby substantially canceling the interconnection. That is, the inverter 20 has an interconnection protection function.

FIG. 2 is a block diagram showing the configuration of the inverter control unit 23 shown in FIG. The inverter control unit 23 includes a differential amplifier (error amplifier) 231, 233, a multiplier 23.
2. A feedback control system having a PWM pulse generation unit 234 and a bandpass filter 235 for extracting a reference frequency component of the system voltage, and a plurality of pulse widths adjusted so that the output state of the inverter 20 becomes appropriate. The set of PWM pulses is output to the inverter main circuit 22 as a switching signal.

The PWM pulse generator 234 includes a phase inversion circuit 241, a triangular wave signal generation circuit 242, and a comparator 2.
43, 244, a logic circuit 245, and a driver circuit 246 for switching drive.

The inverter control unit 23 generates an input error signal Sa indicating the difference between the DC input voltage (output voltage of the solar cell) V1 varying according to the amount of solar radiation and the reference voltage Vref. A current command value signal Si indicating a control target value is generated by multiplication with the signal Sb corresponding to the fundamental frequency component of the system voltage. That is, the magnitude (amplitude) of the output current of the inverter 20 is set by the input error signal Sa, and the phase of the output current is set by the system voltage.

Then, the current command value signal Si is corrected and appropriately amplified in accordance with the signal Sc indicating the phase of the actual output current, and then the current command value signal Si is set in the PWM pulse generator 234. And its phase inversion signal and 20k
A predetermined PWM pulse is generated by comparison with the modulating triangular wave signal Sf of about Hz and an appropriate logical operation.

As a result, in the inverter 20,
Input voltage constant control is performed to adjust the output current so that the output voltage V1 of the solar cell 10 matches the reference voltage Vref.

As the reference voltage Vref, the solar cell 10 is used.
In order to use the generated electric power of 1 as efficiently as possible, the output electric power of the solar cell 10 is set to a value close to the optimum operating voltage (here, 200 V).

That is, in FIG. 4 showing an example of the output characteristics of the solar cell 10, when the amount of solar radiation changes, the output characteristics of the solar cell 10 change like curves CV1 to CV1. In these curves CV1 to CV3, the optimum operating point (also called the maximum power point) changes like P1 to P3, so that the inverter 2
At 0, it is desirable to perform maximum power point tracking control so that the solar cell 10 always operates at the optimum operating points P1 to 3 even when the amount of solar radiation changes. However, as can be understood from FIG. 4, since the change in the voltage (optimal operating voltage) at the optimal operating points P1 to P3 is small, the output voltage V1 of the solar cell 10 is set to the optimal operating voltage in order to simplify the control circuit. The reference voltage Vref is set so as to be a constant voltage (for example, 200 V) near the center within the fluctuation range of.

FIG. 3 is a block diagram showing the configuration of the booster circuit 40 of FIG. The booster circuit 40 includes a chopper unit 50 having a known circuit configuration and a chopper control unit 60 that controls switching of the chopper unit 50.

In the chopper section 50, during the period Ton during which the semiconductor device 53 for switching is on, current flows from the solar cell 10 into the reactor 52 through the smoothing capacitor 51, and during the period Toff during which the semiconductor device 53 is off. , The electric charge energy stored in the reactor 52 before that is transferred to the inverter main circuit 22 side through the output section including the diode 54 for backflow prevention and the resistor 55 for discharging.

The boosting ratio (V2 / V1) in the chopper section 50 is expressed by the equation (1). V2 / V1 = (Ton + Toff) / Toff (1) That is, the ON / OFF cycle T (=
Ton + Toff), the shorter the period Toff, the higher the boost ratio.

On the other hand, the chopper control section 60 is composed of comparators 61, 67 and 68, AND circuits 62 and 69, a driver circuit 63, a differential amplifier (error amplifier) 64, and attenuators 65 and 66.

The comparator 61 compares the output error signal Sp with the above-mentioned triangular wave signal Sf, and the semiconductor device 53
It outputs a logic signal S61 that defines the switching of the.
The output error signal Sp is a signal indicating the difference between the feedback voltage V2a obtained by appropriately reducing the voltage V2 by the attenuator 66 and the preset reference voltage Vz. The logic signal S61 is input to the driver circuit 63 via the AND circuit 62 provided as a signal gate, and is used to generate the switching signal S53 for driving the semiconductor device 53.

As a result, the chopper control section 50 performs PWM switching control for adjusting the pulse width of the switching signal S53 so that the voltage V2 matches a certain upper limit voltage (for example, 450V) corresponding to the boost reference voltage Vz. As a result, the voltage V2 is limited to 450 V even though the output voltage V1 of the solar cell 10 increases almost in proportion to the amount of solar radiation, as shown in FIG.

However, in such PWM switching control, the voltage V1 on the input side is the safety standard upper limit voltage 300V.
This is limited to the case where it is lower and the output side voltage V2 is lower than the withstand voltage 550V of each part in the subsequent stage of the chopper section 40.
That is, the attenuators 65 and 66 and the comparator 6
7, 68 and the AND circuit 69, the voltages V1, V2
When it is detected that at least one of the above is higher than or equal to a predetermined voltage, the output of the AND circuit 62 becomes low level (non-active), and the operation of the booster circuit 40 is substantially stopped.

Next, the normal operation of the inverter 20 having the above configuration will be described. First, it is assumed that the amount of solar radiation is such that the output characteristic of the solar cell 10 becomes the curve CV2. Then, the chopper control unit 60 sets the output voltage V2 to a constant value (4
It is assumed that the PWM control is performed so that the solar cell 10 is at 50 V) and that the solar cell 10 is operating at the operating point a. At this time, the PWM control is performed by the inverter control unit 23 so that the input voltage V1 becomes a constant value (200V). Therefore, a current is supplied from the booster circuit 40 to the inverter main circuit 22 in order to maintain the voltage V1. .

In such a state, the amount of solar radiation decreases and the output characteristic of the solar cell 10 changes from the curve CV2 to the curve CV.
Suppose it changed to 3. If the inverter control unit 23 does not perform any control, the operating point of the solar cell 10 shifts from the point a to the point b, but since the input voltage constant control is performed, the operating point is on the curve CV3. b1 points. This is because the inverter main circuit 22 acts as an impedance converter.

Then, the output voltage V2 of the booster circuit 40 naturally changes due to the change in the pulse width due to the constant input voltage control. The booster circuit 40 performs PWM control so that the output voltage V2 does not change. At this time, when viewed from the booster circuit 40 side, the inverter main circuit 22 is a load, but the impedance of the inverter main circuit 22 changes when the inverter control unit 23 performs PWM control. Since the change of the impedance of the inverter main circuit 22 changes the input voltage V1 of the booster circuit 40, the booster circuit 40 is also PWM.
Take control. As a result, the output voltage V2 of the booster circuit 40 also changes, so that the inverter control unit 23 also performs PWM.
Take control.

By repeating the above series of operations, the control of the booster circuit 40 and the inverter main circuit 22 finally converges, and is stabilized in the state where the above-mentioned constant voltage is maintained. Further, when the amount of solar radiation increases and the output characteristic of the solar cell 10 changes from the curve CV2 to the curve CV1, PWM control is performed so that the operating point is the c1 point, as described above.

According to the above-described embodiment, the step-up ratio of the step-up circuit 40 is not fixed, but is adjusted so as to limit the input voltage V2 of the inverter main circuit 22 in the subsequent stage to 450 V. Therefore, an electrolytic capacitor having a maximum withstand voltage of 550V can be used as the smoothing capacitor C0, and the component price of the smoothing capacitor C0 can be significantly reduced compared to the case where a special electrolytic capacitor having a withstand voltage of more than 550V is used. it can. Further, the withstand voltage of the semiconductor devices Q1 to Q4 of the inverter main circuit 22 can be similarly reduced, and the switching loss can be minimized.

In the above embodiment, it is not necessary to limit the limit value of the output voltage V2 of the booster circuit 40 to 450V.
An appropriate value between the minimum value (about 400 V) required to obtain 200 Vrms by the inverter main circuit 22 and the withstand voltage 550 V of the smoothing capacitor C0 (for example, 5 V).
00V).

In the above embodiment, when the output voltage V2 of the booster circuit 40 is 0 to 450 V, the boosting ratio is a constant value N regardless of the output voltage V1 of the solar cell 10, and the voltage V1 is (450 / N). When it exceeds, the step-up ratio may be set to a value smaller than N to suppress the rise of the voltage V2.

In the above-described embodiment, the chopper type booster circuit 40 may be replaced with another type of switching regulator such as a DC-DC converter. Further, the circuit configuration of the inverter control unit 23, the type of elements of the inverter main circuit 22 and the like can be variously changed in accordance with the gist of the present invention.

[0044]

According to the present invention, at the time of transformerless interconnection with a commercial power system, it is possible to reduce the required withstand voltage of electronic components that constitute an inverter, and to realize a cost reduction and downsizing of a solar power generation system. can do.

According to the second aspect of the invention, the safety of the photovoltaic power generation system can be improved while reducing the required breakdown voltage of the electronic component.

[Brief description of drawings]

FIG. 1 is a block diagram showing a state in which a photovoltaic power generation system according to an embodiment of the present invention is connected to a commercial power grid.

FIG. 2 is a block diagram showing a configuration of an inverter control unit in FIG.

3 is a block diagram showing a configuration of a booster circuit shown in FIG. 1. FIG.

FIG. 4 is a graph showing an example of output characteristics of a solar cell.

FIG. 5 is a graph showing the relationship between the amount of solar radiation and the input / output voltage of the booster circuit.

FIG. 6 is a circuit diagram showing a configuration of a single-phase bridge type inverter main circuit.

FIG. 7 is a block diagram showing a connection form between an inverter and a single-phase three-wire line according to a conventional control method.

[Explanation of symbols]

 1 Solar power generation system 2 Commercial power system 10 Solar cell 22 Inverter main circuit 40 Booster circuit V1 Solar cell output voltage V2 Inverter main circuit input voltage

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location H02M 7/48 F 9181-5H M 9181-5H

Claims (2)

[Claims] 1. A solar cell that outputs DC power, and a booster circuit that boosts the DC power by switching control.
A control method for a photovoltaic power generation system having an inverter main circuit for converting the DC power after boosting to AC power, which is used in connection with a commercial power system, wherein the output power of the solar cell is Pulse width modulation control is performed on the inverter main circuit so that the output voltage becomes almost maximum, and when the input voltage of the inverter main circuit exceeds a certain voltage lower than the withstand voltage, the input voltage of the inverter main circuit A method for controlling a photovoltaic power generation system, comprising adjusting a boosting ratio of the boosting circuit so as to suppress a rise. 2. The method for controlling a photovoltaic power generation system according to claim 1, wherein when the input voltage of the inverter main circuit exceeds the withstand voltage, the booster circuit is put into an operation stop state.